The present invention relates to an optical sheet to be used for a liquid crystal display.
A back light unit comprising a light source and a member for collecting a ray emitted from the light source in a screen of a liquid crystal display is incorporated in the liquid crystal display. More specifically, the ray emitted from the light source are guided to the screen of the liquid crystal display by a light guiding plate provided adjacently to the light source and other optical sheets so that the screen can be displayed clearly.
FIG. 12 shows a schematic structure of a conventional back light unit 40 according to an example.
In FIG. 12, an arrow A indicates a longitudinal direction, an arrow B indicates a transverse direction and an arrow C indicates a vertical direction. As shown in FIG. 12, the back light unit 40 comprises a lamp 41 to be a light source, a light guiding plate 42 provided such that the lamp 41 is positioned on the outside of a left end thereof, a light diffusing sheet 43 to be an optical sheet provided on the upside of the light guiding plate 42, and a prism sheet 44 to be an optical sheet provided on the upside of the light diffusing sheet 43.
In the back light unit 40, the ray incident on the light guiding plate 42 from the lamp 41 is emitted as a ray having a distribution having a peak in a direction forming a specific angle obliquely upward and rightward from the upper surface of the light guiding plate 42, and is then incident on the light diffusing sheet 43. Then, the ray incident on the light diffusing sheet 43 is emitted as a ray having a distribution taking a peak in a direction closer to an upper part from the upper surface through the diffusion during the transmission in the light diffusing sheet 43 and is incident on the prism sheet 44.
Thereafter, the ray incident on the prism sheet 44 is emitted as a ray having a distribution taking a peak in a direction closer to an uppermost part from the upper surface of the prism sheet 44 through a prism portion 44a. Consequently, the ray emitted from the upper surface of the prism sheet is collected into the screen of the liquid crystal display provided in a just upper portion which is not shown so that the screen is illuminated.
In the conventional back light unit, moreover, two prism sheets 44 are superposed and incorporated such that the prism portions 44a are orthogonal to each other and light can be collected into the front face of the screen in the liquid crystal display in some cases.
In the prism sheet 44, furthermore, the corner portion of the prism portion 44a is easily damaged physically by other members. In some cases, therefore, a light diffusing sheet is further provided on the prism sheet to prevent the damage.
In the case in which the light diffusing sheet is used in the conventional back light unit, the ray transmitted from the light diffusing sheet can be emitted as a ray which approach as a just upward direction to be the direction of the front face of the screen in the liquid crystal display if the diffusion of the light diffusing sheet is more increased.
However, when the diffusion of the light diffusion sheet is increased too much, a quantity of light guided to a liquid crystal screen is decreased even if the direction of the peak of the ray can approach the just upward direction to be intended. Consequently, the efficiency of the light source is reduced.
In the case in which the prism sheet is used, the ray cannot be always incident on the prism sheet in a proper direction. For this reason, it is impossible to emit a ray taking a peak of a distribution in almost the just upward direction to be the direction of the front face of the liquid crystal screen. Thus, the liquid crystal screen cannot be illuminated efficiently.
In recent years, moreover, it has been desirable that the size of a product to be the application of the liquid crystal display should be reduced and the size of the back light unit should be reduced.
It is an object of the present invention to provide an optical sheet which can guide a ray to more approach a just upward direction to be a direction of a front face of a screen of a liquid crystal display without reducing an efficiency for a light source and which is not contrary to the request of a reduction in a size.
In order to solve the above-mentioned problem, the present invention provides an optical sheet in which a ray having a distribution taking peak in an obliquely upward direction forming a specific angle with respect to a lower surface thereof is incident from one of sides in a transverse direction, a base portion on a lower side including the lower surface and a fitting portion on an upper side including an upper surface are formed to be fitted each other in a vertical direction with less margin, and the base portion and the fitting portion are formed of materials having different refractive indices, the base portion is provided with a large number of triangular portions having a triangular sectional shape in parallel, the triangular portion having a first side surface positioned on one of the sides and a second side surface positioned on the other side, the first side surface has an angle formed in the transverse direction which is equal to or greater than an angle formed between a direction taking a peak of a ray refracted from the lower surface of the base portion and incident on an inside of the base portion and the transverse direction, and a direction taking a peak of a ray refracted and incident on the fitting portion through the second side surface of the base portion more approaches a just upward direction than a direction taking a peak of a ray transmitted to the second side surface through the inside of the base portion.
According to the optical sheet of the present invention, the ray having a distribution taking a peak in the direction forming the specific angle which is incident through the lower surface can be emitted, from the upper surface, as a ray taking a peak in a direction closer to the just upward direction. Consequently, the ray emitted from a light source can be guided efficiently in the direction of a front face of a liquid crystal screen.
The optical sheet can be formed such that the refractive index of a material forming the fitting portion is set to be smaller than that of a material forming the base portion and the gradient of the second side surface has a greater angle formed in the transverse direction than a gradient perpendicular to a direction taking a peak of a ray transmitted to the second side surface through the inside of the base portion.
In the case in which the refractive index of the fitting portion is set to be smaller than that of the based portion, the second side surface can be formed such that the gradient thereof is almost perpendicular to the transverse direction.
In the optical sheet in which the gradient of the second side surface is almost perpendicular to the transverse direction, the refractive index of the base portion and that of the fitting portion can be mutually adjusted such that the ray having a distribution taking a peak in the specific direction to be a direction forming a specific angle of incidence and incident through the lower surface of the optical sheet can be emitted as a ray having a distribution taking a peak in a direction forming a specific output angle.
In the optical sheet according to the present invention, the ray taking the peak in the direction forming the specific angle of incidence can be converted into a direction of travel of the ray taking a peak in the direction forming the specific output angle. By a combination with a prism sheet in which a ray taking a peak in the direction forming the specific output angle is an optimum direction for incidence, a ray taking a peak in almost the just upward direction can be emitted through the prism sheet so that the ray can be efficiently guided to the liquid crystal screen.
Moreover, in the optical sheet in which the refractive index of the fitting portion is smaller than that of the base portion and the gradient of the second side surface is almost perpendicular to the transverse direction, a difference between the refractive index of the base portion and that of the fitting portion can be set to be 0.15 or more. In the optical sheet having such a structure, the ray having a distribution taking a peak in the specific direction and incident from the lower surface of the optical sheet can be guided upward with respect to the optical sheet as a ray taking a peak in a direction closer to the just upward direction. By using two or more optical sheets without using the prism sheet, accordingly, the ray taking a peak in a direction closer to the just upward direction to be the direction of the front face of the liquid crystal screen can be emitted upward with respect to the two or more superposed optical sheets.
Consequently, it is not necessary to use the prism to guide the ray in the direction of the front face of the liquid crystal display. Therefore, a member constituting a back light unit can be decreased so that the size of the back light unit can be reduced.
In the case in which the difference between the refractive index of the base portion and that of the fitting portion in the optical sheet is set to be 0.15 or more, the refractive index of the base portion can be set to be 1.57 or more. Consequently, the ray incident on the two or more combined optical sheets from below can be emitted as a ray having a distribution taking a peak in almost the just upward direction upward with respect to the two or more combined optical sheets. Thus, the ray can be guided to the liquid crystal screen more efficiently.
Furthermore, in the optical sheet in which the refractive index of the fitting portion is smaller than that of the base portion and the gradient of the second side surface is almost perpendicular to the transverse direction, a difference between the refractive index of the base portion and that of the fitting portion can be set to be 0.3 or more. In the optical sheet having such a structure, the ray having a distribution taking a peak in the specific direction and incident from the lower surface of the optical sheet can be guided upward with respect to the optical sheet as a ray taking a peak in a direction closer to the just upward direction. By using only one optical sheet without using the prism sheet, accordingly, the ray taking a peak in a direction closer to the just upward direction to be the direction of the front face of the liquid crystal screen can be emitted upward with respect to the optical sheet.
Consequently, it is not necessary to use the prism sheet to guide the ray in the direction of the front face of the liquid crystal display. Furthermore, one optical sheet according to the present invention is enough. Therefore, a member constituting a back light unit can be decreased so that the size of the back light unit can be reduced.
In the case in which the difference between the refractive index of the base portion and that of the fitting portion in the optical sheet is set to be 0.3 or more, the refractive index of the base portion can be set to be 1.6 or more. Consequently, the ray incident on the optical sheet from below can be emitted as a ray having a distribution taking a peak in almost the just upward direction upward with respect to the optical sheet. Thus, the ray can be guided to the liquid crystal screen more efficiently.
Moreover, the base portion of the optical sheet in which the refractive index of the fitting portion is set to be smaller than that of the base portion and the gradient of the second side surface is almost perpendicular to the transverse direction can be formed of a thermoformable plastic material. Thus, the base portion to the foundation of the optical sheet can be formed by thermoformation. Consequently, the manufacture can be carried out easily. A polycarbonate based resin can also be used as the thermoformable plastic material for the formation of the base portion. With the polycarbonate based resin, a film having a high transparency can be obtained.
Furthermore, the fitting portion can be formed of an acryl based resin. An acryl based resin of a radiation polymerization type can be used for the acryl based resin. Consequently, the fitting portion can be formed through radiation easily.
It is possible to use an acryl based resin of the radiation polymerization type containing a fluorine group. By using the acryl based resin containing the fluorine group, the refractive index of the fitting portion can be reduced easily.
Moreover, the optical sheet can also be provided with a light diffusing layer on the upper surface of the fitting portion.